This invention relates to the transmission of electrical power over substantial distances wherein energy is transmitted through the atmosphere as a high frequency electromagnetic wave. More specifically, this invention relates to combined antenna and rectifier structure for receiving high frequency electromagnetic energy and converting the received high frequency energy to a direct current (DC) electrical signal suitable for use in conventional distribution grids and/or other apparatus that requires electrical energy.
Various proposals have been presented for the transmission of energy between a source and a remote location that requires such energy wherein the energy is converted to a radio frequency signal that is transmitted as an electromagnetic wave and received at the remote location. Because of efficiency considerations, the frequency of the transmitted signal of such a system is generally in the microwave region and is converted to a low frequency or direct current signal that is more amenable for use with conventional energy storage apparatus (e.g., batteries) and a wide variety of conventional electrically powered apparatus at the receiving station. Examples of proposed power distribution systems of this type include those intended to supply energy to remote facilities located on the surface of the earth; those intended to, in effect, beam energy from earth to airborne vehicles, including conventional aircraft space vehicles and satellites; and those intended to distribute power from a large master space satellite or "mother-ship" to smaller subsatellites.
Although advances in the state of the art relative to other types of power distribution systems have provided more satisfactory alternative means for accomplishing each of the above-mentioned goals, the need to obtain energy from sources other than petroleum and other nonrenewable deposits has created renewed interest in transmission of electrical power by microwave radiation. More specifically, one proposal under consideration is the use of a synchronous satellite for collecting solar energy wherein the satellite also includes means for converting direct current electrical energy provided by solar cells into high frequency electromagnetic waves that are retransmitted and beamed to the earth. This technique has several advantages over converting solar energy that is received at the surface of the earth into electrical energy. For example, the electromagnetic energy provided by such a satellite can supply energy to the earth that is not limited to normal daylight hours. Of perhaps even greater importance, the electromagnetic energy is subjected to far less attenuation than the light and heat content of solar energy that propagates through the earth's atmosphere, including substantial cloud cover that frequently blankets various portions of the earth's surface.
Because of those constraints inherent to all applications in which power is to be transferred between remotely located points through transmission of microwave energy and additional constraints imposed by a satellite transmission system of the above-mentioned type, a need arises for specially structured receiving arrangements and associated means for converting the received electromagnetic energy into a direct current electrical signal. In this regard, many of the prior art proposals have included one or more rectifiers that are associated with each antenna element that receives the electromagnetic energy. Such an arrangement would be extremely costly and complex in that even when the satellite uses the most directional antenna array possible, usable electromagnetic energy will impinge on at least several square miles of the earth's surface. To efficiently collect this energy with an array of antennas wherein one or more rectifier diodes is associated with each antenna would, of course, require an extremely large number of diodes, especially since small antennas such as half-wave dipoles have been utilized in the vast majority of the prior art arrangements.
The use of a prior art arrangement of the above-described type also presents a disadvantage or drawback in that it is not possible to achieve maximum efficiency with such an arrangement. In particular, the electromagnetic field intensity and hence the power density is not uniform throughout the irradiated region, but varies from a maximum value near the center of the irradiated region to a substantially lower value along the outer periphery thereof. In this regard, variations on the order of 10 db will often be encountered. This means that an antenna positioned in the central portion of the irradiated region will provide an electrical signal at a greater power level than will an identical antenna which is positioned a substantial distance from the central region. Since the efficiency of semiconductor rectifiers varies substantially with power level, this also means that an antenna combination that is optimized for operation in one portion of the irradiated region will not operate efficiently within a second portion of the electromagnetic field.
Another problem that is encountered in utilizing prior art arrangements in such a system is the reradiation of harmonically related energy. In this regard, it is well-known that nonlinearities in the operating characteristics of typical RF-DC diode conversion systems cause generation of electrical signals that are harmonically related in frequency to the frequency of the received electromagnetic signal. Prior art antenna-rectifier apparatus are not arranged to effectively prevent such harmonic signal components from radiating into the earth's atmosphere from the antenna elements and the associated interconnecting transmission lines. Because of the significant signal levels present within a solar power satellite system, such radiation not only constitutes a loss of energy that can potentially contribute to the DC output of the system, but can also cause other problems such as interference with various communications systems.
The problems associated with filtering the DC signal supplied by the semiconductor rectifiers can also be more significant in the type of system under consideration. For example, in accordance with most proposals, the irradiated regions are located in sparsely populated and somewhat remote areas with the rectified signal being carried to the area which will utilize the electrical power via conventional power transmission grids. Thus, AC signal components including harmonically related signals that would be conducted through and radiated from such a grid should be eliminated. As is the case with high frequency radiation from the receiving elements, the prior art does not appear to provide adequate and reliable containment of harmonic energy or prevent its radiation from the DC distribution grid.
In addition to the above-discussed disadvantages and drawbacks, the previously proposed arrangements have not achieved the ease of assembly and servicing that is necessary in a large scale system such as the above-discussed solar transmission system. In this regard, prior art proposals have often utilized an arrangement wherein the diodes are installed in substantially inaccessible portions of an associated antenna or within the dielectric region of transmission line structure leading from an antenna. Further, the relatively few attempts that have been made to install impedance matching structure between the antenna and the semiconductor rectifier or to provide signal filtering generally utilize distributed parameter networks that are located along the transmission path between the antenna and the system output terminals. Often, these previous attempts at impedance matching and filtering through use of distributed parameter networks employ components formed on the same dielectric substrate that supports the conductive antenna elements. Thus, these networks are relatively complex, are not easily serviced, cannot be fully tested or calibrated prior to final assembly and often radiate undesirable harmonic signals since they are not electromagnetically shielded.
Accordingly, it is an object of this invention to provide combined antenna-rectifier structure especially suited for use in a large scale power distribution system wherein a substantial region is radiated with microwave energy.
It is another object of this invention to provide antenna-rectifier structure that is readily adaptable for efficient operation within electromagnetic fields exhibiting substantial variation in field strength or power density.
It is yet another object of this invention to provide combined antenna-rectifier structure that includes provision for efficient impedance matching and filtering of the associated electrical signals to thereby minimize radiation from the antenna elements thereof.
It is still another object of this invention to provide a combined antenna-rectifier structure of the above-described type wherein the rectifier, impedance matching and filtering networks can be easily installed to the antenna structure and can be easily removed for replacement or servicing.
Further, another object of this invention is to provide a modular antenna-rectifier structure of the above-described type in which the RF-to-DC conversion efficiency, harmonic leakage and DC output ripple of each modular unit can be easily determined prior to incorporation of such a module into final large scale array of such modules.